This invention relates to optical instruments and in particular to optical instruments having an exit pupil at which an image of an object may be viewed by an observer.
In conventional optical instruments the size of the exit pupil is determined by a function of the numerical aperture of the instrument and the overall magnification of the instrument and hence the size of the exit pupil is of fixed and relatively small dimension. Consequently it is necessary for an observer to accurately align the entrance pupil of his eye with the exit pupil of the optical instrument in order properly to view an image or otherwise receive light from the instrument.
In my pending European patent application No. 94905777.2 I have described and claimed optical apparatus provided with an enlarged exit pupil whereby an observer is able to enjoy the freedom to receive images or light from the optical apparatus by placing his eye pupil anywhere within an enlarged exit pupil. The enlarged exit pupil is obtained by the provision of a diffractive element located at an intermediate focal or image plane of the optical instrument. The diffractive element comprises a substrate having a plurality of parallel first grooves in a surface of the substrate, edges of the first grooves being coincident and forming lines of a first diffractive grating and a plurality of parallel second grooves in the surface of the substrate, the second grooves extending perpendicular to the first grooves and edges of the second grooves being coincident and forming lines of a second diffractive grating. While a construction of diffractive element as described in my pending European patent application No. 94905777.2 produces an enlarged exit pupil, my present invention provides an improved optical instrument having an enlarged exit pupil.
According to one aspect of the invention an optical includes means to produce an optical image to be viewed at an exit pupil by an observer; means to produce an enlarged exit pupil comprising an array of a plurality of said exit pupils; said means to produce the array of exit pupils comprising a diffractive element located at an intermediate focal or image plane of the optical instrument; said diffractive element including substrate bearing a plurality greater than two of optical diffractive gratings, each said optical diffractive gratings comprising a set of parallel diffraction lines defined by a set of parallel grooves in a surface of said substrate; the parallel diffraction lines of each diffractive optical grating of the plurality of optical diffractive gratings extending angularly relative to the diffraction lines of other optical diffractive gratings of the plurality of optical diffractive gratings.
According to a second aspect of the invention a diffractive element of an optical instrument includes a substrate; a surface on said substrate; a plurality greater than two of optical diffractive gratings; said optical diffractive gratings each comprising a set of parallel diffraction lines defined by a set of parallel grooves in said surface of said substrate; the parallel diffraction lines of each diffractive optical grating of the plurality of optical diffractive gratings extending angularly relative to the diffraction lines of other optical diffractive gratings of the plurality of optical diffractive gratings.